1. Field of Invention
The invention relates to an ink jet apparatus.
2. Description of Related Art
Among non-impact printers that have expanded their market by supplanting existing impact printers, ink jet printers are simplest in principle and easily realize color printing as well as printing in multiple gradations. Particularly, drop-on-demand type ink jet printers, which eject ink droplets for printing, are rapidly becoming widespread because of their excellent ejection efficiency and low running costs.
Typical drop-on-demand type ink jet printers include a Kyser type disclosed in U.S. Pat. No. 3,946,398 and a thermal jet type disclosed in U.S. Pat. No. 4,330,787. However, the Kyser type is difficult to miniaturize, while the thermal jet type requires heat-resistant ink because intense heat is applied thereto.
To simultaneously overcome the above-mentioned problems, U.S. Pat. No. 4,879,568 proposes, as a new system, a shear mode type printer utilizing piezoelectric ceramics.
FIGS. 12 and 13 show an exemplary sectional view of a shear mode type ink jet head. The printhead 600 includes an actuator substrate 601 and a cover plate 602. Formed in the actuator substrate 601 are a plurality of ink channels 613 shaped like a narrow groove and extending perpendicularly to the sheet as shown in FIG. 12, and a plurality of dummy channels 615 carrying no ink. The ink channels 613 and the dummy channels 615 are isolated by sidewalls 617. A sidewall 617 is interposed between each ink channel 613 and each dummy channel 615. The sidewalls 617 are composed of upper walls 609 and lower walls 611, which are polarized in directions P1 and P2, respectively. The directions P1 and P2 are opposite to each other and parallel to the height direction of the side walls 617.
A nozzle 618 is provided at one lengthwise end of each of the ink channels 613. Provided on the other end is a manifold for supplying ink. The dummy channels 615 are closed at the manifold-side ends to block the entry of ink and do not have a nozzle at the other end. Electrodes 619, 621 are provided, as a metal layer, on opposite side surfaces of each of the sidewalls 617. More specifically, two adjacent sidewalls 617, 617 are separated by an ink channel 613, and electrodes 619, 619, 621, 621 are provided on opposite side surfaces of the two adjacent sidewalls 617, 617 to constitute one set of actuators. Each electrode 619 provided on the internal surface of the sidewalls 617, 617 of each of the ink channels 613 is grounded. Electrodes 621, 621, each provided on the side surface facing an associated dummy channel 615, are connected to an associated output circuit 34 (FIG. 4) that generates drive signals.
Upon application of a voltage to two adjacent electrodes 621, 621 on sidewalls 617 separated by an ink channel 613, the upper and lower walls 609, 611 of the two adjacent sidewalls 617, 617 deform, by a piezoelectric shearing effect, in such directions that the volumetric capacity of each of the ink channels 613 increases. More specifically, as shown in FIG. 13, when an ink channel 613b is driven, a voltage of E [V] is applied to two adjacent electrodes 621c, 621d, which are separated by the ink channel 613b, while the electrodes 619 of ink channel 613b are grounded. Consequently, electric fields are generated on sidewalls 617c, 617d in the directions E, and the upper and lower walls of the side walls 617c, 671d deform, by a piezoelectric shearing effect, in such directions that the volumetric capacity of the ink channel 613b increases. At this time, the pressure within the ink channel 613b, including in the vicinity of the nozzle 618b decreases. By maintaining such a state for a period of time required for a pressure wave to propagate, one way, along the ink channel 613b, ink is supplied from the manifold (not shown) for that period of time T.
The one-way propagation time T represents a time required for a pressure wave in the ink channel 613b to propagate longitudinally along the ink channel 613b, and is given by an expression T=L/c, where L is a length (perpendicular to the sheet of FIG. 13) of the ink channel 613b, and c is a speed of sound in the ink within the ink channel 613b. 
Based on the theory of propagation of a pressure wave, upon expiration of the time T after the application of a voltage of E [V], the pressure in the ink channel 613b is reversed to a positive pressure. Concurrently with the reversing of the pressure, the voltage applied to the electrodes 621c, 621d are reset to 0 [V].
Then, the sidewalls 617c, 617d return to their original states, as shown in FIG. 12, and pressurize the ink. The pressure reversed to a positive pressure in addition to the pressure generated upon returning of the sidewalls 617c, 617d generates a high pressure in the vicinity of the nozzle 618b of the ink channel 613b. As a result, an ink droplet is ejected from the nozzle 618b. 
If a time period between application and resetting of the voltage of E[V] does not agree with the one-way propagation time T, energy efficiency for ink ejection decreases. Particularly, when the time period between application and resetting of the voltage is even multiplies of the one-way propagation time, no ink is ejected. Normally, when the time period between application and resetting of the voltage agrees with the one-way propagation time, energy efficiency reaches its peak, and so does the ink droplet ejection velocity. Thus, the time period between application and resetting of the voltage is preferably odd multiplies of the one-way propagation time.
Recently, demands for higher printing resolutions have increased in order to improve print quality. To respond to such demands, it is preferable to reduce the ink droplet volume. The ink droplet volume is usually reduced by reducing the nozzle diameter or by reducing the drive voltage, that is, the ink droplet ejection velocity.
In the printhead 600, when a nozzle 618 is exposed to air in a non-ejection state for a while, the ink solvent in the vicinity of the nozzle 618 evaporates, and the viscosity of ink around the nozzles 618 increases. Consequently, the ink droplet ejection velocity and the ink droplet volume decrease, and the ink trajectory is curved by a sidewind generated when the printhead 600 travels. As a result, ink droplet striking positions are displaced. Ink droplets as tiny as 20 pl (picoliters) or less in volume, are especially susceptible to such a problem. As one of the conventional methods to solve the above-described problem, when the nozzles have been exposed to air in a non-ejection state for a predetermined time, a higher drive voltage than usual is applied to increase the ink droplet ejection velocity. However, changing the drive voltage for each print command increases the cost of a power source. Further, changing the drive voltage requires extra time and disables high-speed printing.
In view of the foregoing problems, an object of the invention is to provide an ink jet apparatus capable of obtaining excellent print quality, at low cost, without changing the drive voltage.
To achieve the above object, an application time of an ejection pulse is elongated in response to a print command, for at least an initial dot, issued after a nozzle has been kept in a non-ejection state. More specifically, a period of time during which an ejection pulse is applied to an actuator is elongated by widening the pulse width of an ejection pulse or by increasing the number of ejection pulses. By doing so, the volume of an ejected ink droplet is increased, and thus, the ink droplet trajectory becomes unlikely to curve under the influence of the sidewind. Consequently, even when the nozzle has been exposed to air in a non-ejection state for a while, excellent print quality can be obtained without displacement of the ink droplet striking positions.
Although an actuator of the above-described Kyser type, the thermal jet type, or other known types can be used for ejecting ink, it is more preferable to use an actuator of the type in which the volumetric capacity of an ink channel is increased/decreased to generate a pressure wave.
When the time required for a pressure wave to propagate along an ink channel is set as T, the pulse width of an initial ink ejection pulse to be applied to an actuator after the nozzle has been kept in a non-ejection state should be odd multiplies of T. Thereby, energy efficiency is increased more than usual, and the ink droplet ejection velocity is also increased. As a result, the ink droplet trajectory is unlikely to be curved by a sidewind and excellent print quality can be obtained.
Increasing the number of ejection pulses or widening the pulse width can be selectively accomplished by a control device. In the case where printing is performed at various resolutions by changing the ink droplet volume, ink droplets having a volume suitable for a desired resolution can be ejected by increasing the number of ejection pulses or by widening the pulse width, even when the nozzle has been exposed to air in a non-ejection state.
Time elapsed since the nozzle entered a non-ejection state is easily determined by counting, with the use of a timer, the duration of the non-ejection state, or by counting the number of periodically outputted clock signals accompanied by no ejection data.
Further, in a printer that performs printing line by line by shuttling a printhead along the paper, an initial ejection pulse to be applied after a new line has been started can be controlled, in the same manner as described above, by widening the pulse width or by increasing the number of pulses. Thus, even when the nozzle has been exposed to air in a non-ejection state during a line feed operation, or has moved along the paper while being exposed to air, without any ejection data, after a line feed operation, adverse effects on the nozzle can be eliminated.
Further, it is preferable to apply a non-ejection pulse following an ejection pulse in order to cancel the pressure wave vibrations generated by the ejection pulse. This is because, when the ink viscosity is low, ink droplets might be undesirably ejected, or the pressure wave generated by application of the next ejection pulse might be affected by the residual pressure wave vibrations. Thus, the application of a non-ejection pulse enables stable ejection. It also allows the next ejection pulse to be outputted after a very close interval, which enables high-speed printing.
In this case, the crest value of the non-ejection pulse is equal to that of the ejection pulse. The non-ejection pulse should be applied upon expiration of a time period between 2.0T and 2.3T, or more preferably, between 2.1T and 2.2T after the ejection pulse falls. At this time, the pulse width of the non-ejection pulse should be between 0.2 T and 0.65T, or more preferably, between 0.3T and 0.55T.